The present invention relates to a process for the preparation of crystalline microporous and mesoporous metal silicates composed of silicon dioxide and one or more metal oxides. In another aspect, the present invention relates to products obtainable by said process. Still further, the present invention relates to the use of these products.
A variety of crystalline microporous and mesoporous metal silicates are known. According to the IUPAC definition, micropores are understood as meaning pores with a diameter of less than 2 nm, and mesopores are understood as meaning pores with a diameter of 2 to 50 nm. These definitions are used herein.
Some crystalline metal silicates with regular micropores or mesopores are extremely effective catalysts in a very wide variety of applications. In particular, microporous products of the composition (SiO.sub.2).sub.1-x (TiO.sub.2).sub.x, in which titanium atoms replace some of the silicon atoms in the crystal lattice, have achieved industrial importance as oxidation catalysts. The structures of these metal silicates differ according to the starting materials and conditions of preparation. Thus, for example, so-called titanium silicalite-1, titanium silicalite-2 and titanium beta-zeolite have the MFI, MEL and BEA crystal structures, respectively. Crystal structure types of known zeolites and silicalites are described in W. M. Meier, D. H. Olson, Atlas of Zeolite Structure Types, Butterworth-Heinemann, 1993. Known structures with regular mesopores are the MCM-41 and MCM-48 structures described in C. T. Kresge et al., Nature, 359 (1992), pp. 710-712. A survey of structures with regular mesopores is also given in S. Behrens, Angew, Chemie, 1996, 108 (5), pp. 561-564.
It is known that the catalytic efficacy of generic metal silicates is substantially dependent on their phase purity and the morphology and, hence, on the conditions of preparation. For example, the catalytic activity of titanium silicalite is reduced by other titanium-containing phases, for instance TiO.sub.2, and by an increase in the crystal size; see, B. Notari in (a) Catal. Today 18 (1993), p. 163 and (b) Stud. Surf. Sci. Catal., 67 (1991), p. 243.
Generic metal silicates, for instance titanium silicalite-1, can be prepared by hydrothermal synthesis. In the first step, a silicon source and a titanium source, conventionally tetraalkyl orthosilicate and tetraalkyl orthotitanate, and water are condensed to a gel in the presence of a structure-determining quaternary ammonium cation, usually used in the form of the quaternary ammonium hydroxide, and the gel is then crystallized under hydrothermal conditions, usually above 100.degree. C. and under autogenous pressure. The solid that is form is separated off, washed, dried and calcined above 300.degree. C. The way in which the Ti component is introduced is liable to cause problems, it often being impossible to exclude contamination of the product by TiO.sub.2 as a foreign phase--loc. cit. B. Notari (b). According to EP 543 247, the quaternary ammonium hydroxide can be replaced as the template with a combination of a quaternary ammonium salt and a base such as ammonia, although the crystals obtained are relatively large. According to U.S. Pat. No. 5,198,203 a mesoporous titanium silicate of the MCM-41 structure can be prepared with cetyltrimethylammonium hydroxide as the template.
Another known silicon source is pyrogenic silicic acid. According to R. Kumar et al. in Stud. Surf. Sci. Catal., 84 (1994) p. 109, titanium silicalite-1 can be obtained by hydrothermal synthesis from pyrogenic silicic acid and tetrabutyl orthotitanate. The use of a tetraalkyl orthotitanate carries the risk of obtaining a product of insufficient phase purity.
According to EP 0 311 983, titanium silicalites are prepared by impregnating a coprecipitated porous TiO.sub.2 --SiO.sub.2 material, which can be amorphous or crystalline, with a template compound and then subjecting the product to hydrothermal synthesis. Suggestions of using coprecipitates with other metals, or pyrogenic mixed oxides, are not to be found in said document.
In known synthesis processes the generic microporous metal silicates are formed as crystallites with a size usually of less than one micrometer, which can only be separated from a liquid at considerable expenses. For many industrial applications the fine material is therefore coarsened by a subsequent agglomeration step. According to EP 0 265 018, the agglomeration is effected using oligomeric SiO.sub.2 as a binder.
According to EP 0 299 430, a preformed amorphus SiO.sub.2 matrix is impregnated with an aqueous solution containing a soluble titanium compound and a suitable template, and crystallized under hydrothermal conditions, the shape and size of the SiO.sub.2 matrix remaining essentially unchanged. This process again has the disadvantage that the use of a soluble titanium compound carries the risk of reducing the phase purity and hence the catalytic activity.
Accordingly, it is an object of the invention to produce generic metal silicates of high phase purity and high catalytic activity.
Another object of the invention is to provide a process suitable not only for the preparation of products based on (SiO.sub.2).sub.1-x (TiO.sub.2).sub.x, but also for the preparation of products in which titanium is replaced with one or more other metals in the crystal lattice.
Still further, another object of the invention is to carry out the process in such a way that binder-free shaped objects of the crystalline microporous and mesoporous metal silicates can be obtained directly, i.e., without a subsequent agglomeration step.